linux-kernel - Re: [PATCH v3 3/4] cxl, acpi/hmat: Update CXL access coordinates directly instead of through HMAT

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Message-ID: <20250902162450.00002684@huawei.com>
Date: Tue, 2 Sep 2025 16:24:50 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: Dave Jiang <dave.jiang@...el.com>
CC: <linux-cxl@...r.kernel.org>, <linux-acpi@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <gregkh@...uxfoundation.org>,
	<rafael@...nel.org>, <dakr@...nel.org>, <dave@...olabs.net>,
	<alison.schofield@...el.com>, <vishal.l.verma@...el.com>,
	<ira.weiny@...el.com>, <dan.j.williams@...el.com>,
	<marc.herbert@...ux.intel.com>, <akpm@...ux-foundation.org>,
	<david@...hat.com>, <stable@...r.kernel.org>
Subject: Re: [PATCH v3 3/4] cxl, acpi/hmat: Update CXL access coordinates
 directly instead of through HMAT

On Fri, 29 Aug 2025 15:29:06 -0700
Dave Jiang <dave.jiang@...el.com> wrote:

> The current implementation of CXL memory hotplug notifier gets called
> before the HMAT memory hotplug notifier. The CXL driver calculates the
> access coordinates (bandwidth and latency values) for the CXL end to
> end path (i.e. CPU to endpoint). When the CXL region is onlined, the CXL
> memory hotplug notifier writes the access coordinates to the HMAT target
> structs. Then the HMAT memory hotplug notifier is called and it creates
> the access coordinates for the node sysfs attributes.
> 
> During testing on an Intel platform, it was found that although the
> newly calculated coordinates were pushed to sysfs, the sysfs attributes for
> the access coordinates showed up with the wrong initiator. The system has
> 4 nodes (0, 1, 2, 3) where node 0 and 1 are CPU nodes and node 2 and 3 are
> CXL nodes. The expectation is that node 2 would show up as a target to node
> 0:
> /sys/devices/system/node/node2/access0/initiators/node0
> 
> However it was observed that node 2 showed up as a target under node 1:
> /sys/devices/system/node/node2/access0/initiators/node1
> 
> The original intent of the 'ext_updated' flag in HMAT handling code was to
> stop HMAT memory hotplug callback from clobbering the access coordinates
> after CXL has injected its calculated coordinates and replaced the generic
> target access coordinates provided by the HMAT table in the HMAT target
> structs. However the flag is hacky at best and blocks the updates from
> other CXL regions that are onlined in the same node later on. Remove the
> 'ext_updated' flag usage and just update the access coordinates for the
> nodes directly without touching HMAT target data.
> 
> The hotplug memory callback ordering is changed. Instead of changing CXL,
> move HMAT back so there's room for the levels rather than have CXL share
> the same level as SLAB_CALLBACK_PRI. The change will resulting in the CXL
> callback to be executed after the HMAT callback.
> 
> With the change, the CXL hotplug memory notifier runs after the HMAT
> callback. The HMAT callback will create the node sysfs attributes for
> access coordinates. The CXL callback will write the access coordinates to
> the now created node sysfs attributes directly and will not pollute the
> HMAT target values.
> 
> A nodemask is introduced to keep track if a node has been updated and
> prevents further updates.
> 
> Fixes: 067353a46d8c ("cxl/region: Add memory hotplug notifier for cxl region")
> Cc: stable@...r.kernel.org
> Tested-by: Marc Herbert <marc.herbert@...ux.intel.com>
> Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Dave Jiang <dave.jiang@...el.com>

Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>